Last summer’s Nippon TV scandal, in which a producer admitted he’d bribed monitor families into watching his program, has compromised the Japanese ratings system, but no matter how skeptically you regard such numbers the ratings performance of the pop group SMAP during the first month of the new year can’t fail to impress.

In the week ending Jan. 11, NHK’s new Sunday night historical drama “Shinsengumi,” which stars SMAP’s Shingo Katori, dominated the competition with a 26.3 percent share in the Kanto area. That number seemed unbeatable until the following week, which saw the opening episode of Fuji TV’s “trendy drama” “Pride,” starring Katori’s colleague Ta kuya Kimura, take a whopping 28 percent. In fact, during the week ending Jan. 18 the top three rated programs were “Pride,” the variety show “SMAP × SMAP,” and the premiere of the TBS drama series “Suna no Utsuwa (Vessel of Sand),” which stars SMAP leader Masahiro Nakai. Even “Boku to Kanojo to Kanojo no Ikiru Michi,” which stars another SMAPper, Tsuyoshi Kusanagi, premiered with a 19.3 percent share, placing fifth overall for the week.

The popularity of the idol boy group, whose members now range in age from 27 to 31, has ceased to be a phenomenon that demands analysis, and the high ratings were certainly no surprise. However, some people were surprised when it was announced more than a year ago that Katori would star in “Shinsengumi.” NHK’s “taiga drama” is a cultural institution, a yearlong serial about an important figure in Japanese history (“taiga” means “great river,” as in the “river of time”). And while NHK always uses idols in taiga dramas, Katori is the first to play the lead role, at least while still an idol.

Like the commercial stations, NHK wants as big an audience as it can get. In that regard, the show’s writer, Koki Mitani, whose suit is situation comedy and whose heroes are Billy Wilder and Neil Simon, is as important to the project’s success as Katori is.

Taiga drama freaks and historical purists have flooded the Internet with gripes about the series, which is about a squad of young samurai sent to Kyoto to counter antishogunate forces during the period of civil strife just prior to the 1868 Meiji Restoration. Complaints have ranged from the contemporary lilt of the dialogue to factual inaccuracies.

Mitani thinks of himself as an entertainer, and on that count “Shinsengumi” is more successful than most taiga dramas, which tend to get bogged down in court intrigues, power struggles and battle plans. Mitani’s comic ensemble style is well-suited to a story about a bunch of misfits who come together for a cause, and Katori, who has worked with Mitani before, fits the bill.

But he isn’t much of an actor, and while ratings are the reason idols get leading roles in TV dramas while trained pros are relegated to supporting parts, NHK isn’t supposed to care about ratings. For years now, NHK’s relevance as the broadcast standard bearer has been waning, and for better or worse SMAP — or, more accurately, the male idol personality represented by SMAP’s management company, Johnny’s Jimusho — has dominated the TV zeitgeist in Japan. NHK has already announced that another Johnny’s idol, the androgynous Hideaki Takizawa, will star in next year’s taiga drama.

Drama series have become so idol-oriented since the mid-’90s that they are now reconfigured for their stars. “Pride,” which is about a company hockey team, is as formulaic as TV dramas get. It’s completely built around the dramatic personality that Kimura has developed over the years: principled, cool, professional, but with an undercurrent of romantic longing.

“Suna no Utsuwa” is already a famous novel and movie, but in the current TV series, the story is retooled to take advantage of Nakai’s dramatic reputation, which is darker than most idols’. In “Suna,” he plays a concert pianist with a mysterious past who kills a man after the man reveals he knows about that past. Though the book and movie concentrated on the detective who unravels the murder, the TV show is centered on the pianist. Ken Watanabe, basking in his Oscar nomination for “The Last Samurai,” plays the detective, but his participation is a topical afterthought. Nakai is the main event.

The problem with idols is not their poor acting, but rather their ubiquity. Nakai’s penchant for glowering and Katori’s game attempt at masculine purity contrast comically with their respective images as TV personalities. It’s difficult to take them seriously as characters. Since Kimura doesn’t do many variety shows, his dramatic persona is easier to accept, even if it isn’t particularly interesting.

Many people consider Kusanagi SMAP’s best actor, but it’s only because he is treated as one, meaning he adapts himself to a role rather than the other way around. In “Boku to Kanojo . . .” he plays a workaholic father just getting to know his young daughter after his wife leaves him. Though he’s 30, he doesn’t look or act old enough to make the character credible.

No one thinks the character might be better served by a real actor because real acting is as immaterial to Japanese trendy dramas as real directing and real writing is. And “Shinsengumi” isn’t so much an improvement over past taiga dramas, as it is a relief from them. The bottom line of these series is their ability to be marketed. That’s probably why Goro Inagaki, the fifth SMAP member, isn’t in a drama series this season, and hasn’t been since his self-imposed show biz exile in 2002 after a traffic mishap. Unless you’re on top at all times, people seem to forget you’re even there.